A convergence of evidence from autonomous ocean floats (Argo network), satellite observations, and ship-based measurements indicates that the Southern Ocean carbon sink has weakened substantially over the past two decades. The ocean absorbs roughly 25% of human-emitted CO2, and the Southern Ocean alone accounts for approximately 40% of total oceanic CO2 uptake.
The weakening is attributed to increased wind-driven upwelling of carbon-rich deep water (bringing previously sequestered CO2 back to the surface), warming-driven stratification reducing deep mixing, and possible saturation effects as surface waters approach equilibrium with the rising atmospheric CO2 concentration.
If the trend continues, a greater fraction of future emissions will remain in the atmosphere, potentially accelerating warming beyond current model projections that assume a relatively stable ocean sink.